Drug delivery devices allowing for multiple dosing of a required dosage of a liquid medicinal product, such as liquid drugs, and further providing administration of the liquid to a patient, are as such well-known in the art. Generally, such devices have substantially the same purpose as that of an ordinary syringe.
Drug delivery devices of this kind have to meet a number of user specific requirements. For instance in case of those with diabetes, many users will be physically infirm and may also have impaired vision. Therefore, these devices need to be robust in construction, yet easy to use, both in terms of the manipulation of the parts and understanding by a user of its operation. Further, the dose setting must be easy and unambiguous and where the device is to be disposable rather than reducible, the device should be inexpensive to manufacture and easy to dispose. In order to meet these requirements, the number of parts and steps required to assemble the device and an overall number of material types the device is made from have to be kept to a minimum.
Typically, the medicinal product to be administered is provided in a cartridge that has a moveable piston or bung mechanically interacting with a piston rod of a drive mechanism of the drug delivery device. By applying thrust to the piston in distal direction, a predefined amount of the medicinal fluid is expelled from the cartridge.
Due to inevitable manufacturing tolerances there may for instance persist axial clearance between a cartridge's piston and the piston rod. Typically, prior to a primary use of the device, an end-user has to conduct a so-called priming of the drive mechanism in order to ensure, that already with an initial dose setting and a first subsequent dose dispensing step, an accurate amount of the medicinal product is disposed in a predefined way.
Since a self-administering user might be physically infirm, it is desirable to simplify or even to eliminate the need for such a user-conductible priming procedure.
Document EP 1 911 479 A1 discloses for instance a dose dispensing device, wherein a piston and a piston rod are electromagnetically or magnetically coupled, and wherein the piston and piston rod comprise a magnetic effect generating member.
Since the generation of an electromagnetic field requires a supply of electric energy, such a solution is generally not applicable to drug delivery devices of pen-injector type.
Insofar, known solutions feature the common drawback, that for elimination of axial clearance between piston rod and piston, the piston rod has to be axially shifted by its associated drive mechanism. Axial clearance- and backslash elimination implies to bring the piston rod in direct abutment position with a cartridge's piston. Such axial displacement of the piston rod for the purpose of clearance elimination is regarded as disadvantageous, because it typically involves a respective actuation of dose setting or dose dispensing means by the user.
It is therefore an object of the present invention to provide a drive mechanism for a drug delivery device featuring improved and facilitated clearance and manufacturing tolerance elimination. It is a further object of the invention to redundantize a priming procedure to be conducted by the end user. The invention further focuses on improvements related to patient safety and intends to simplify the general device handling. It is a further object of the invention, to provide a drive mechanism for a drug delivery device with clearance eliminating means being inexpensive to produce and being easy to assemble. Finally, it is an object of the invention to provide a method of eliminating clearance in a drive mechanism of a drug delivery device.